In recent years, nanotechnology, which by forming a substance at the nanometer size, has the object of obtaining different physical properties from the bulk substance or molecular level substance, is attracting much attention. On the other hand, because of enhanced awareness of alternatives to petroleum resources and environmental awareness, applications of reproducible natural fibers are also gathering much attention.
Among natural fibers, cellulose fibers, and especially cellulose fibers derived from timber (pulp) are widely used, mainly as paper products. The width of the cellulose fibers used in paper is mostly from 10 to 50 μm. The paper (sheet) obtained from these types of cellulose fibers is opaque, and being opaque, is widely used as paper for printing. On the other hand, if the cellulose fibers are treated (beaten or ground) using a refiner, a kneader, or a sand grinder or the like, thereby refining (microfibrillating) the cellulose fibers, then a transparent paper (such as a glassine paper) can be obtained. However, the transparency of these transparent papers is at the semi-transparent level, the optical transparency is low compared with polymer films, and the degree of cloudiness (haze value) is also large.
Further, cellulose fibers have a high elastic modulus and are aggregates of cellulose crystals having a low coefficient of thermal expansion, and by forming a composite of cellulose fibers with a polymer, the thermal dimensional stability is improved, enabling use in laminates and the like. However, typical cellulose fibers are crystal aggregates, and are fibers having cylindrical voids, and therefore there is a limit to the dimensional stability.
A water dispersion of finely fibrous cellulose having a fiber width of 50 nm or less, produced by mechanically grinding cellulose fibers, is transparent. On the other hand, a finely fibrous cellulose sheet contains voids, and therefore causes white irregular reflections that increase the opacity, but if a finely fibrous cellulose sheet is impregnated with a resin, then the voids are filled, meaning a transparent sheet can be obtained. Moreover, the fibers of a finely fibrous cellulose sheet are aggregates of cellulose crystals, and are extremely rigid and have a small fiber width, and therefore compared with a typical cellulose sheet (paper), the number of fibers within a sample of the same mass is dramatically higher. As a result, by forming a composite with a polymer, the fine fibers are dispersed more uniformly and densely within the polymer, and the thermal dimensional stability improves dramatically. Further, because the fibers are fine, the transparency is high. Finely fibrous cellulose composite sheets having these types of properties are attracting extremely high levels of expectation as flexible transparent substrates (transparent substrates that can be bent and folded) for organic EL and liquid crystal displays.
Numerous refining techniques and techniques for forming composites with polymers have been disclosed in relation to finely fibrous cellulose, but there have been almost no disclosures of techniques for forming finely fibrous cellulose into composite sheets while maintaining good industrial productivity.
Specifically, Patent Documents 1 to 3 disclose techniques for converting cellulose fibers to a finely fibrous form, but there is no disclosure nor mention relating to a technique for converting the finely fibrous cellulose to sheet form while simultaneously forming a composite with a polymer.
Patent Documents 4 to 10 disclose techniques and the like for improving the physical properties such as the mechanical strength by forming a composite of finely fibrous cellulose with a polymer, but there is almost no disclosure concerning techniques for simplifying the compositing process.
Furthermore, Patent Documents 10 to 20 disclose techniques for converting finely fibrous cellulose to a sheet form, but these techniques are unable to achieve an industrial level of productivity, and therefore the provision of a simple method of compositing finely fibrous cellulose with a polymer to form a composite sheet is desirable.